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Model Coaching to Create a Collection System Model

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Model coaching is the term we used to define the process by which a consultant teaches a client how to construct and use a collection system model by making the client part of the modeling team. This process was used to construct a hydraulic computer model of the collection system for the Pima County Regional Wastewater Reclamation Department (PCRWRD or County) collection system. PCRWRD serves about 370 square miles in the Tucson, Arizona Metropolitan Area with about 3,500 miles of sanitary sewers.

The County needed a model of their collection system to identify capacity issues, including peak wet weather flows, and for future planning. The County did not want the modeling process to follow the pattern they had seen with other modeling projects: a consultant builds a model, the consultant trains the County over a few days on using the model, the County struggles to learn how to use the model on their own and eventually abandons the effort. The County wanted a dynamic tool that they felt comfortable using and for which they could claim ownership.


To accomplish the goal of creating a model, the County and Brown and Caldwell worked closely together to construct the model. The County allocated two staff to work full-time on the modeling. Brown and Caldwell then trained the two County modelers over the course of a year. One Brown and Caldwell engineer was the primary trainer with the part-time assistance of another engineer. The model training started as an initial multi-day training course to become familiar with the modeling software. Over the next 12 months, the training continued with monthly multi-day training sessions and frequent phone calls, emails, and web conferences. To aid in project communication, a project website was set up. On the website, project communications and data could be stored and tracked.

The training covered the entire modeling process. This included incorporating pipe and manhole data from a Geographic Information System (GIS), performing flow metering to capture dry and wet weather data, calculating dry and wet weather flows, calibrating and validating the model to the measured flow data, and using the model for analysis and planning in the system.

The County's commitment to learn modeling was evident because of the allocation of two fulltime staff. Two full-time staff would not be necessary in all cases (e.g. with small systems), but it worked very well for this project. The primary modeler is an engineer with a good background in hydraulics and in the County's collection system. The second modeler is an expert in the County's GIS system. For this project, his expertise was necessary to clean up the GIS data and integrate new manhole survey data that came in through the course of the project.


Setting up the project schedule and budget for the model coaching was different than setting up a usual modeling project. Budgeting was more complicated because it was difficult to assess the skills of the County modeling staff before beginning the project. It was also difficult to set a schedule when most of the project progress was dependent on the client. To get around these difficulties, a very detailed scope of work was developed which defined the responsibilities of the County and the consultant. These responsibilities included the specific portion of each modeling task that was to be accomplished by each party. The scope of work also discussed adjusting the project schedule and fee if the County could not keep its commitment to make available the full-time modelers.


The model coaching approach was very successful. Prior to using the model coaching approach, PCRWRD spent several years attempting to create a model using modeling software and then using a spreadsheet model. Those efforts did not give PCRWRD a useful capacity analysis tool. However, through the model coaching, a model was created that the County continues to use for planning and engineering decisions.

After the completion of the initial model training, PCRWRD has taken full ownership of the model. The two County modelers have continued to maintain, update, and use the model. Because the modelers were trained on all aspects of the model during its development, they understand the capabilities and limitations of the model. They also understand the data they need to collect to continue to improve the model and refine its accuracy in the coming years. County engineers have been consulting with the County modelers on questions related to the collection system and the modelers have been able to provide timely answers. As the County engineers continue to learn the capabilities of the model, they will rely more on the model and less on old planning data.


Because the County has a good understanding of the modeling process, they are starting to integrate the model into many of their capacity and planning processes. The County is starting to use the model for the following processes:

Capital Improvement Plan Updates

The previous capital improvement plan (CIP) was created based on a spreadsheet model. The County is using the new computer model to update the CIP. With this new dynamic tool, the County has been able to question planned projects and is starting to re-prioritize their CIP.

Short-Term Planning

The County is revamping their short-term planning process to use the model results. Previously, the short-term planning group relied on temporary flow metering to find capacity issues. With only flow metering data at discrete locations, the planners could not identify all of the potential capacity issues when planning for new development. But with the model, they can see the effects of new development on all of the collection system piping from the point of collection down to the wastewater treatment plants.

Long-Term Planning

The County is also using the model for longer-term planning. The model has helped the County to focus on a process to plan for long-term growth. This process has included the steps that the County modelers learned as they built the computer model. These steps have included defining the long-term service boundary, defining where each drainage basin will connect to the existing collection system, and how to calculate flows for the future service area.

Inflow and Infiltration

The County modelers recognize that the model can be used to identify potential areas of inflow and infiltration (I/I). Identifying areas for wet weather I/I reduction in the Tucson area is difficult because significant rainfall that causes major I/I issues may happen only a few times each year. With limited rainfall and a limited number of flow meters, the County is unable to monitor the entire system and identify all of the major locations of I/I. By using the existing flow metering data in conjunction with the model, the County will be able to more easily identify areas of potential I/I. The model will help the County to identify strategic locations for placing flow meters to capture the rainfall when it occurs.

Examples of Model Uses

Following are a few specific examples of where the County has used the model since the completion of the training:

The County currently has a project to construct a large diameter gravity pipeline to connect the two major WWTPs. The model was used to verify the peak flow to one of the plants for the year 2030. This peak flow was provided to the design engineers for sizing the plant interconnection pipeline.

The pipeline connecting the WWTPs will relieve an existing interceptor that is at capacity. The model was used to identify alternatives to relieve the interceptor in the short-term before the plant interconnection is constructed. The alternatives included replacing sections of a pipeline, adding wet weather storage, and sending flows to another interceptor.

While reviewing the existing CIP, the County modelers noticed that a project in the CIP may not be necessary. The model showed that the area previously thought to have capacity issues is surcharging due to backwater from another area with insufficient capacity.

County has constructed two out of three phases of a new interceptor. The model was used to find the year when the third phase will be required, or the year when the existing pipes will be at capacity. Based on the model results, the County planned the third phase so it would be constructed before the existing pipes reach capacity.


The model coaching process was very successful for the County. The County accomplished its goal of building a working hydraulic model that they have confidence in using. The model coaching was successful because of the County's commitment and investment in learning the details of the modeling process.
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Keywords: Hydraulic model; model coaching; training; wastewater collection systems

Document Type: Research Article

Publication date: 2009-01-01

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